1. Field of the Invention
This invention relates to image processing, particularly to the processing of color images displayed on a cathode ray tube for reproduction by a color plotter or color printer. The invention also relates to an improved circuit in color image processing systems for separating synchronization signals from composite video input signals and for generating accurate timing signals. Furthermore, the invention relates to an improved method for processing color images acquired by such a system to enable their more realistic reproduction by a plotter.
2. Description of the Prior Art
As is well known in computer graphics, a series of graphics display terminals may be connected through graphics controllers to a channel controller. The channel controller in turn is coupled to a host central processing unit. The channel controller enables the host central processing unit to be connected to the individual graphics controllers associated with the various display terminals. In this manner, commands entered on the graphics display terminal are processed by the controllers and supplied through the channel controller to the host CPU in accordance with protocols determined by the host CPU. In one type of image processing system of the prior art, when a hard copy of an image displayed on a graphics display terminal was desired, special software in the host CPU was used to control driver circuits, channel adapters, and rasterizing controllers, all of which in turn drove a plotter. The system required a large number of expensive components and was generally cumbersome. In addition, it operated relatively slowly and provided images substantially inferior to those obtainable using photographic equipment.
Prior art systems for acquiring hard copies of color images displayed on video monitors divide themselves into several categories. One technique, rather than creating the image on the screen, or in random access memory for controlling the screen, uses suitable software to reproduce the information in another random access memory. This allows use of the information by the host CPU to drive the plotter directly. Unfortunately, because of the vast number of different types and configurations of systems, such an approach is highly hardware-dependent, requiring emulation of the system hardware by software in the host CPU before transmitting the image to the plotter. Furthermore, such an approach requires considerable time to produce each image, often a half hour or more.
Another technique of providing hard copies, particularly suited to video complying with RS170 standard such as television monitors and 512-line interlaced displays, is the use of dry silver processes, for example, using equipment manufactured by Tektronics. Such equipment, functioning primarily as analog equipment, effectively copied the contents of the scan lines onto dry silver paper. Such systems are substantially limited, however, by their inability to render grey tones and their resolution. Furthermore, such systems are unable to process video information at a sufficiently high speed and are not suited to color reproduction.
Another technology was to display the video on the inside of a film printer box, then copy it photographically using film. A spinning color wheel was employed to provide color reproduction.
Yet another prior art approach is the Seiko D scan approach. In this system the video is digitized into a frame buffer, then sent to a plotter for reproduction. Only one bit per pixel is digitized and that bit is not further processed before driving the plotter. As a result the image is not half-toned.
In synchronization separators used in video image processors of the prior art, numerous stages are typically employed to separate the synchronization pulses from the video information. Each of these stages introduces an additional undesirable delay into the signal processing and skews the system timing. Such synchronization circuits typically did not employ feedback to lock onto the synchronization pulse and separate it from the composite video.